Solid electrolytes, also known as fast ion conductors, are materials that can function as solid state ion conductors and can be used, for example, in solid oxide fuel cells and lithium ion batters. In a lithium ion battery, for instance, lithium ions move from a negative electrode to a positive electrode during discharge (and back when charging) via the solid electrolyte. The solid electrolyte, such as lithium aluminum titanium phosphate (LATP), can conduct lithium ions through vacancies in the LATP crystal lattice. In lithium ion batteries, the solid electrolyte membrane can also provide a hermetic barrier between the anode and the cathode, which can prevent the anode and cathode from sharing a common electrolyte solution.
The ability to produce dense, conductive lithium ion electrolyte membranes is thus important to the development of lithium ion batteries. Various challenges may exist during the manufacture of such membranes, including manufacturing a membrane having sufficient density to be hermetic while still providing sufficient conductivity and economy. Conventional processes for producing hermetic membranes, for example, glass-ceramic processes, can produce dense, hermetic membranes, but often at the expense of other attributes such as conductivity and cost. Glass-ceramic processes can also be challenging because the starting composition is limited to those that can form a stable glass, thus limiting the glass-ceramic route to specific compositions (e.g., the glass-forming regions).
Glass-ceramic processes can also be limited by high operating temperatures, which can be in excess of 1000° C. The sintering of ceramic lithium ion electrolyte materials to a density sufficient to produce a hermetic membrane can be difficult due to vaporization of volatile lithium and/or phosphate species at temperatures greater than 1000° C. These limitations, in addition to restricting control of the process conditions, can also restrict the compositional space available for providing enhanced properties, such as conductivity and environmental stability.
To address these and other issues, Applicant previously disclosed a reactive sintering method for forming dense, hermetic electrolyte membranes. This method is disclosed in U.S. Patent Application Publication No. 2013/0137010, which is incorporated herein by reference in its entirety. The reactive sintering method involves combining reactive powders and heating them to simultaneously react the components and densify the reaction product. For instance, an amorphous, glassy, or low melting temperature solid reactant can be combined with a refractory oxide reactant to form a mixture, which can then be cast as a green body and reactively sintered.
The reactive sintering method can offer significant advantages over prior art glass-ceramic methods, for example, the ability to prepare a wider variety of compositions with higher conductivity and/or density. However, Applicant has discovered that the reactive sintering process may still have one or more disadvantages. For example, because electrolyte membranes can be very thin, e.g., less than 200 microns or even less than 100 microns, the performance of these thin membranes can be sharply impacted by inhomogeneity, such as localized pockets of inadequately mixed components or components that have settled out of the mixture. Furthermore, membranes produced according to the reactive sintering method may suffer from wrinkling and/or rupturing during and/or after firing, which can be caused, for instance, by organic materials in the green body and/or adhesion of the green body to setter plates during firing. A creep flattening step, e.g., annealing the membrane under a weight, may thus be required to produce a final product with sufficient flatness, which of course can add to the complexity and/or cost of the manufacturing process.
Accordingly, it would be advantageous to improve upon the reactive sintering methods previously disclosed by Applicant to provide hermetic electrolyte membranes with improved density, homogeneity, and/or flatness. It would also be advantageous to provide methods for making such membranes at lower temperatures, which can result in lower cost and/or improved process control.